Multi-year genetic sampling indicates maternal gene flow via colony emigrations in the army ant Eciton burchellii parvispinum

Abstract

Sex-biased dispersal occurs when one sex disperses more frequently or farther than the opposite sex. In ants, dispersal is often male biased and males typically contribute more strongly to gene flow within and among ant populations. However, army ants may offer an exceptional case of female-biased dispersal, because army ant colonies (particularly species in the Neotropical genus Eciton) routinely emigrate among nest sites. We hypothesized that maternal mobility via successive colony emigrations would reduce the relative male bias in dispersal, and that emigrations would contribute to gene flow within an Eciton population. We genotyped workers (highly variable DNA microsatellite loci) from Eciton burchellii parvispinum colonies collected over a 10 × 10 km area in Monteverde, Costa Rica. Samples were spaced approximately 3 years apart (in 2006 and 2009), the typical time between colony reproductive bouts. We used worker genotypes to reconstruct the genotypes of their mother queens and of a subset of the males that sired the workers. We then analyzed the population genetic structure of the queen and male genotypes by comparing individual-based relatedness estimates for each sex across geographic distance and over time. There was no significant difference in spatial genetic structure between the sexes, either within or between the 2006 and 2009 samples. There was some evidence against genetic philopatry: 2009 queens were significantly unrelated to 2006 queens collected nearby (within 0.5 km). The patterns suggest maternal dispersal via emigrations contributes to gene flow, reducing or eliminating male biases in dispersal. From two populations, we estimated average queen dispersal distance to be 0.63–1.04 km, a greater distance than many winged ant queens disperse. Because emigrations over the lifetime of army ant queens may contribute to gene flow across the landscape, habitat connectivity should be maintained to permit colony emigrations and support genetic diversity in populations of this keystone species.